It is often desirable to administer two medical liquids to a patient in sequence. This sequential administration can be accomplished by providing a medical liquid administration set which includes two liquid sources (a primary source and a secondary source) connected through a Y-connector from which a single tube leads through an injection needle to the patient. A tube leading from the primary source or set to the Y-connector incorporates a check valve for stopping the flow of liquids from the primary source (and for preventing backflow of liquids from the secondary source to the primary source) when the pressure at the valve from the secondary source is greater than the pressure at the valve from the primary source. A medical administration set useful for sequential administration of liquids is described in U.S. Pat. No. 4,354,492, which issued on Oct. 19, 1982 to McPhee. U.S. Pat. No. 4,354,492 is incorporated herein by this reference.
In the type of medical administration sets described in the above-referenced patent, the secondary source or set is normally positioned at a higher elevation than the primary set. When in this configuration, liquid flows from the secondary set to the patient until the liquid level in the secondary set falls to the level of the liquid in the primary set. At this time, the check valve in the tube leading from the primary set to the Y-connector opens, thereby allowing liquid to flow from the primary set to the patient.
The provision for automatic commencement of flow from the primary set to the patient ensures that there will be no interruption in the flow of liquids to the patient. This is important because such an interruption can, for example, result in blood-clotting in the injection needle, requiring needle replacement and attendant patient discomfort.
When a fine control of the amounts of the liquids being administered to the patient is desired, a positive displacement infusion pump, such as one of the pumps provided by Kendall McGaw Laboratories, Inc. of Irvine, Calif., under the trademarks "521 INTELLIGENT PUMP" or "522 INTELLIGENT PUMP," can be incorporated into the administration system. When such a pump is used, it is connected in the system in the line between the Y-connector and the patient. The check valve in the line between the primary set and the Y-connector serves to provide for automatic sequencing of liquid delivery from the pump. For example, as was the case for the above-described gravity system, when the liquid level in the secondary set falls to the level of the liquid in the primary set as it is being pumped to the patient, the check valve opens, and liquid automatically begins to be pumped from the primary set to the patient.
Pumps such as the McGaw "521" and "522" pumps include a cassette that is connected in the line between the Y-connector and the injection needle. The cassette, which is installed in the pump housing prior to operation of the system, comprises flexible chambers into which liquid flows during the pump intake stroke, and out from which liquid is forced during the pump output stroke. Prior to placing such a pump into service, the flexible cassette and the associated inlet and outlet tubing are filled with liquid from one of the liquid sources (usually the primary set) to eliminate entrapped air from the system. This process fills the entire administration set with liquid and results in excess liquid being in the cassette chambers. After the system is filled, the cassette is placed into the pumping compartment in the pump housing, and the pump housing is closed with the cassette inside. Closure of the pump housing tends to force excess liquid contained in the cassette into the lines upstream from the pump. The displaced fluid builds up pressure between the pump and the check valve, which creates an overpressure condition. The pumps are designed with an overpressure alarm and will not start unless a differential pressure of at least a selected value between the pump intake and outlet is sensed. In some instances, this can be about 75 inches of water pressure. The overpressure condition that results from closing the overfilled cassette into the pump housing is sufficient to set off the overpressure alarm and prohibit the pump from starting.
It is therefore desired that an administration set be provided which incorporates a flow-sequencing check valve that is configured to eliminate overpressure conditions which result in pump overpressure alarms and malfunctioning.